Temperature endpointing of chemical mechanical polishing

ABSTRACT

A method is described for temperature endpointing of a CMP process. A temperature sensor ( 110 ) detects temperature changes when the CMP polishing process transitions between different materials.

CROSS-REFERENCE TO RELATED PATENT/PATENT APPLICATIONS

[0001] The following commonly assigned patent/patent applications are hereby incorporated herein by reference: U.S. Pat. No./Ser. No. Filing Date TI Case No. 09/034,514 03/04/98 TI-23590AA

FIELD OF THE INVENTION

[0002] The present invention relates to a method of detecting the endpoint of a chemical mechanical polishing method. The endpoint method involves the real time monitoring of the temperature change of the polishing pad as the polisher transitions from one material to another.

BACKGROUND OF THE INVENTION

[0003] Integrated circuits are comprised of semiconductor devices which are fabricated on silicon wafers and/or substrates. These semiconductor devices are interconnected on the wafer by forming conducting lines or interconnects that contact the various terminals of the device. Currently, these interconnects are formed using aluminum, copper, doped polycrystalline silicon and other electrically conducting metals and alloys. The complexity of integrated circuits requires a number of different layers or levels of interconnects. The various layers of interconnect are separated from each other by dielectric insulator layers. In most cases these insulator layers are formed using silicon dioxide, silicon nitride or some combination of these materials. The semiconductor devices on the wafer are electrically connected to the interconnects through the use of contacts. In forming these contacts, an insulating layer is first formed on the wafer completely covering the semiconductor device. Using standard photolithography, contact windows are opened in the insulating layer to expose the device terminals to which electrical contact is to be made. A barrier layer is first formed in the contact window to reduce unwanted diffusion between the device terminal and the conducting material that will subsequently be used to fill the contact window. This barrier layer is sometimes formed using titanium nitride or materials with similar properties. Following the formation of this barrier layer, a metal layer is formed which completely fills the window and covers the surface of the insulating layer. This metal layer typically comprises tungsten, aluminum, titanium, or a metal with similar properties. To complete the formation of the contact, the portion of the metal layer that is above the insulating layer is removed. In addition to the formation of contacts, the above described process is used to electrically connect the various layers of interconnect by forming structures usually referred to as vias. In the case of both contact and via formation, the metal used to fill the contact window openings must be removed from all areas of the underlying insulating layer. Typically this process is performed using a selective plasma etch process. Given the very small feature size of current integrated circuits, selective plasma etching will not produce satisfactory results and chemical mechanical polishing (CMP) is now used to perform this metal removal.

[0004] In the chemical mechanical polishing (CMP) process, a rotating polishing head or wafer carrier, is typically utilized to hold the wafer under controlled pressure against a rotating polishing platen. A chemical slurry is controllably introduced between the polishing head and the wafer to facilitate the polishing. The polishing process is a combination of mechanical abrasion and a chemical reaction. A particular problem encountered during the CMP process is the control of the various process parameters to achieve the desired wafer characteristics. In using the CMP process to remove the metal from the underlying insulating layer, it is crucial that the polishing process is stopped as soon as all the metal is removed and a planar surface is formed. Over polishing will lead to erosion of the dielectric layer particularly in dense contact or via arrays while under polishing will result in remaining metal. In both cases, the presence of these defects will often lead to circuit failure or unreliable circuit performance. It is therefore important to be able to precisely detect the endpoint of the polishing process. Current methods of endpoint detection involve using a timed process, monitoring the current of the polishing motor, and monitoring the temperature of the wafer. These methods often lead to unsatisfactory results. Thus there is a need for an accurate endpoint detection method that is easily implementable on existing CMP equipment. The present invention is directed to a novel method for controlling a CMP process in real time by monitoring the temperature changes of the polishing pad. This process is especially useful for the tungsten CMP processes used in the formation of contacts and vias and for copper metal processes.

SUMMARY OF INVENTION

[0005] The instant invention described (CMP end point method) is independent of the pattern density of the contacts and vias being formed. It is also independent of film thickness and most other processing conditions. It is easily adaptable into existing CMP equipment and is non invasive. The instant invention can also be applied to the removal of multiple films or layers. Other technical advantages will be readily apparent to one skilled in the art from the following FIGUREs, description, and claims. In particular an embodiment of the method comprises: providing a semiconductor substrate with a top film of a first material overlying a second film of a second material; monitoring temperature of a polishing pad during a chemical mechanical polishing of said top film of said first material; and stopping said chemical mechanical polishing upon detecting a non random change in temperature of said polishing pad as said chemical mechanical polishing transitions from said top film of said first material to said second film of said second material.

BRIEF DESCRIPTION OF THE DRAWINGS

[0006] For a more complete understanding of the present invention and the advantages thereof, reference is now made to the following description taken in conjunction with the accompanying drawings, wherein like reference numerals represent like features, in which:

[0007]FIG. 1 is an schematic diagram of a CMP polishing apparatus.

[0008]FIG. 2 is a cross-section diagram of the wafer surface topography, polishing pad, slurry, and the wafer carrier.

[0009]FIG. 3 is a cross-section diagram showing the end of the CMP process.

[0010]FIG. 4 is a cross-section diagram showing the instant invention applied to a plurality of films.

DETAILED DESCRIPTION OF THE INVENTION

[0011] Illustrated in FIG. 1 is a schematic diagram of a CMP system for use with the end point detection method of the instant invention. The system comprises a table (platen) 10 that rotates around the axis 30 in the direction shown 40. In an embodiment of the instant invention, the platen 10 rotation is in a counter clockwise direction. A polishing pad 20 is affixed to the platen and is used to polish the surface of the semiconductor wafer or semiconductor substrate 90. In some instances the polishing pad 20 will be formed using polyurethane or other similar materials. The semiconductor wafer 80 is attached face down to a wafer carrier 50 that rotates around an axis 70. The wafer carrier 50 rotates in the same direction as the platen 10. The slurry 105 used in the polishing process is delivered to the polishing pad through a slurry delivery system 100. In an embodiment of the instant invention, additives 106 can be added to the main slurry using a separate delivery system 112. The chemicals in this case will combine on the polishing surface. In another embodiment of the instant invention, the chemicals can be combined above the surface of the table by mixing the slurry and the additives before delivery to the platen surface or polishing pad 20. The centrifugal force on the slurry due to the platen rotation causes mixing of the chemicals and their delivery to the wafer or substrate surface 90. In an embodiment of the invention, during the polishing process, the wafer carrier periodically lifts the wafer off the surface of the platen (and polishing pad) to allowing mixing of the chemicals (interpolish lift off). For example in a particular process this might occur about every 30 seconds for about 3 seconds. The frequency and duration of this interpolish lift off process is not confined to these times but will be a function of the process where CMP of different materials will require different times or frequencies. In most CMP systems the pad temperature is monitored using a temperature sensor 110. In an embodiment of the instant invention this temperature sensor comprises an infra red sensor. Such a temperature sensor provides an electrical signal that is proportional to the temperature of the pad 20 and the slurry 105.

[0012] Shown in FIG. 2 is a cross section of the wafer carrier 50, the semiconductor wafer or semiconductor substrate 80 and the polishing pad 20. The wafer or substrate 80 comprises silicon devices (not shown for clarity), dielectric layers 120, 130, interconnects 140, and a metal film 150 which will be used to form the vias for contacting the interconnects 140. In an embodiment of the instant invention, the metal film 150 will be tungsten and or a tungsten alloy or copper, and the dielectric layers 120 and 130 will be silicon dioxide or silicon nitride. As shown in FIG. 2, the portion of the metal film 150 underlying the dielectric film 130 will be removed during the CMP process. During polishing, the wafer 50 is affixed to the wafer carrier 50 with the wafer surface to be polished 155 facing the polishing pad 20. In an embodiment of the invention, a tungsten layer 150 will be in contact with the slurry 105 and the polishing pad 20. Typically, the slurry 105 used during the process will comprise alumina or another abrasive silica and hydrogen peroxide or a suitable metal oxidizing agent. A suitable slurry is a colloidal silicia formulation. Suitable slurries include, but are not limited to, Cabot (El Dorado Hills, Calif. USA)SS-W2000. This slurry represent the mainstream slurry used in metal polish operations by most major US semiconductor manufacturers. In the preferred embodiment of the present invention, the slurry is SS-W2000 from Cabot. During the CMP process, the slurry will first oxidize the surface of the metal film 155 and the resulting metal oxide will be removed by the mechanical polishing action of the pad 20 and the abrasive agents in the slurry. During this polishing process, the temperature sensor 110 will monitor the slurry 105 and the pad 20 whose temperature will remain constant within the tolerances of the CMP equipment. As stated above, during the polishing process both the polishing pad 20 and the wafer carrier 50 will rotate in the same direction. This rotation produces friction between the oxidized metal surface 155 and the pad 20 which aids the polishing process. The polishing process ends when the metal film 150 underlying the dielectric layer 130 has been removed and a planar dielectric surface exists.

[0013] The end point of the CMP process and the resulting planar surface 160 is illustrated in FIG. 3. The removal of the underlying metal film 150 results in the formation of vias 157 which will provide electrical contact to the interconnects 140. Typically, additional processing will be performed following the CMP process to produce the completed integrated circuit. As shown in FIG. 3, at the end point of the metal film removal CMP process, the surface 160 that is in contact with the slurry 105 and the polishing pad 20 will have changed from metal 150 and/or metal oxide to a dielectric 130. This change in surface type changes the interaction between the slurry 105 and the wafer surface 160 at the end point of the process. Compared to the oxidization and polish of the metal film (150 in FIG. 2), the dielectric film 130 undergoes a purely mechanical polish with a resulting decrease in pad and slurry temperature. This temperature decrease is detected by the temperature sensor 110 and the resulting change in the electrical signal provided by the temperature sensor 110 is used to end point the CMP process. If the metal film 150 is tungsten and the dielectric film 130 is silicon dioxide, the resulting decrease in temperature of the pad 20 and slurry 105 is approximately 5 degrees centigrade.

[0014] An advantage of the above described CMP end point method is that it is independent of the pattern density of the contacts and vias being formed. It is also independent of film thickness and most other processing conditions. It is easily adaptable into existing CMP equipment and is non invasive. The instant invention can also be applied to the removal of multiple films or layers. In such an embodiment, any number of films could be removed and the CMP process would end point on the material transition that produced the desired polishing pad temperature change. A multiple film CMP process is illustrated in FIG. 4. Here, the wafer or substrate 80 is attached to the wafer carrier 50 which rotates during polishing. The multiple films 170, 175, 180, and 185 can comprise any combination of metal, dielectric, semiconductor, and insulator films. In particular the films 170, 180, and 185 could comprise silicon oxide and silicon nitride layers. During the CMP polishing process, the temperature of the pad 20 and the slurry 105 will be monitored using a temperature sensor 110. As the CMP polishing process transitions from one layer to the next, the temperature of the pad 20 and the slurry 105 will change in a non random manner. This non random change in temperature can be used to end point the CMP process after the desired number of layers have been removed. The idea of a non random temperature change in the context of the instant invention is a temperature change that can be differentiated from the normal random temperature variations of the pad 20 and slurry 105 that occurs during the CMP polishing of any single layer or film.

[0015] Although the instant invention has been described with respect to a metal-dielectric CMP process, it can be used for any CMP process that results in a transition between two different materials. Some of these processes include the formation of contacts, vias, pre-metal 1 dielectric (PMD) layer, interlevel dielectric (ILD) layer, formation of metal trenches in damascene and dual damascene CMP processes, and shallow trench isolation (STI) CMP processes. While this invention has been described with reference to illustrative embodiments, this description is not intended to be construed in a limiting sense. Various modifications and combinations of the illustrative embodiments, as well as other embodiments of the invention will be apparent to persons skilled in the art upon reference to the description. It is therefore intended that the appended claims encompass any such modifications or embodiments. 

We claim:
 1. A method for detecting the end point of a chemical mechanical polishing process comprising: providing a semiconductor substrate with a top film of a first material overlying a second film of a second material; monitoring temperature of a polishing pad during a chemical mechanical polishing of said top film of said first material; and stopping said chemical mechanical polishing upon detecting a non random change in temperature of said polishing pad as said chemical mechanical polishing transitions from said top film of said first material to said second film of said second material.
 2. The method of claim 1 wherein said top film is a metal.
 3. The method of claim 1 wherein said second film is a dielectric.
 4. The method of claim 1 wherein said monitoring temperature of said polishing pad is performed using an infrared detector.
 5. The method of claim 1 wherein said top film is a metal selected from the group consisting of tungsten and copper.
 6. The method of claim 5 wherein said second film is silicon dioxide.
 7. The method of claim 1 wherein said top film is silicon oxide.
 8. The method of claim 7 wherein said second film is silicon nitride.
 9. A method for detecting the end point of a chemical mechanical polishing process comprising: providing a semiconductor substrate with a top film of a first material overlying a second film of a second material; monitoring temperature of a slurry during a chemical mechanical polishing of said top film of said first material; and stopping said chemical mechanical polishing upon detecting a non random change in temperature of said slurry as said chemical mechanical polishing transitions from said top film of said first material to said second film of said second material.
 10. The method of claim 9 wherein said top film is a metal.
 11. The method of claim 10 wherein said second film is a dielectric.
 12. The method of claim 9 wherein said monitoring temperature of said polishing pad is performed using an infrared detector.
 13. The method of claim 9 wherein said top film is selected from the group consisting of tungsten and copper.
 14. The method of claim 13 wherein said second film is selected from the group consisting of silicon dioxide and silicon nitride.
 15. A method for detecting the end point of a chemical mechanical polishing process comprising: providing a semiconductor substrate with a plurality of films; monitoring temperature of a polishing pad during a chemical mechanical polishing of said plurality of films; and stopping said chemical mechanical polishing upon detecting a non random change in temperature of said polishing pad as said chemical mechanical polishing transitions from a first material film of said plurality of films to a second material film of said plurality of films.
 16. The method of claim 15 wherein said plurality of films is formed from material from the group consisting of tungsten, silicon dioxide, copper, silicon nitride, and silicon.
 17. The method of claim 15 wherein said monitoring temperature of said polishing pad is performed using an infrared detector. 